While it's possible to design the track first and make the layout after, I think a lot of people will design gameplay spaces with a payload track in mind. However, you might fall into the track of making the map one large corridor with no pacing (as tends to be the way of a lot of bad payload maps)

To avoid that, try designing a bunch of smaller sections of gameplay space with a bit of track running through them and then joining them all together to make a complete map. I think Barnblitz was designed this way, as the control points are all located at the end of an open area.

There's a lot more strategies you can use for designing a good payload map, but I'm still trying to figure them out.